A portrait photo.

Alex Elzenaar

I (he/him) am currently in the School of Mathematics at Monash University supervised by Jessica Purcell. I usually work in between the areas of geometric group theory, geometric topology, and metric geometry. I am particularly interested in modern classical geometry (for instance as studied by Coxeter and Thurston) and relationships with other branches of mathematics (knot theory, number theory, algebraic geometry, complex dynamics...). I am also interested in visualisation of mathematical objects (including art) and the study of writing (mathematical or otherwise).

Email: alexander.elzenaar@monash.edu ▫ Curriculum VitaePublications

This all came as a shock to Sanshirō, and he struggled to grasp what kind of pressure a beam of light could have and what function such pressure could possibly serve.

[…]

Crowds gathered before "Woman in Forest" from the day the show opened. The bench turned out to be a useless ornament—although tired spectators would sit there in order not to see the picture. But even while they rested, some exchanged views on "Woman in Forest".

– Sōseki Natsume, Sanshirō (translated by Jay Rubin), p.20 and p.226. Penguin (2009).
Older quotes

Oppose hundreds of job cuts at Victoria University of Wellington (and thousands nationwide)

Where is your rage now? by Emma Maguire

Mathematical videos that I like: Coxeter on Escher's Circle LimitNot KnotMathematics as metaphorPhyllotaxisHow to write mathematics badlyHyperbolic VRThe Riley sliceKnots don't cancelMixingKaleidoscopesHyperbolica by CodeParadeThree-dimensional geometriesJulia set capacitorsPolyrhythmsQuasicrystalsSymmetryGromovMagic square

Other links and videos: some sculptures around PōnekeHilma af Klint at the City Gallery in 2021-2022Robin White: Making of That VaseEnergy Work: Kathy Barry/Sarah Smuts-Kennedy"Rita" by Quentin AngusPatrick Pound at City Gallery Wellington ▫ Len Lye: A Colour Box, Colour Cry, KaleidoscopeA Painter's Journey: Rita Angus' Central OtagoSolving Pale FireThe fiction of BorgesDan Olsen on creative inadequacyString theory

Look at these other people: Ari Markowitz, \( \mathbb{H}^3 \) group action visualiser and Bruhat-Tits tree visualiser ▫ work of David Groothuizen Dijkema

Publications and preprints

  1. Research announcement: Alex Elzenaar, Gaven Martin, and Jeroen Schillewaert. "On Thin Heckoid and Generalised Triangle Groups in \( \mathrm{PSL}(2,\mathbb{C}) \)", 2024. arXiv: 2409.04438 [math.GR].
  2. Preprint: Alex Elzenaar, Jianhua Gong, Gaven Martin, and Jeroen Schillewaert. "Bounding deformation spaces of 2-generator Kleinian groups", 2024. arXiv:2405.15970 [math.CV].
  3. Preprint: Alex Elzenaar and Shayne Waldron. "Putatively optimal projective spherical designs with little apparent symmetry", 2024. arXiv:2405.19353 [math.CO].
    Associated dataset: A.E. and S.W. A repository of spherical \( (t,t) \)-designs (Version 2). Zenodo, 2022. DOI: 10.5281/zenodo.8126277
  4. Proceedings article: Alex Elzenaar, Gaven Martin, and Jeroen Schillewaert. "Concrete one complex dimensional moduli spaces of hyperbolic manifolds and orbifolds". In: 2021-22 MATRIX annals. Ed. by David R. Wood, Jan de Gier, Cheryl E. Prager, and Terrence Tao. MATRIX Book Series 5. Springer, 2024, pp. 31–74. DOI: 10.1007/978-3-031-47417-0_2. Preprint version: arXiv:2204.11422 [math.GT]. Corrected preprint: PDF.
  5. Journal article: Alex Elzenaar, Gaven Martin, and Jeroen Schillewaert. "The combinatorics of the Farey words and their traces." In: Groups, Geometry, and Dynamics. Accepted, to appear. DOI: 10.4171/GGD/832. Preprint version: arXiv:2204.08076 [math.GT].
  6. Journal article: Alex Elzenaar, Gaven Martin, and Jeroen Schillewaert. "Approximations of the Riley slice." In: Expositiones Mathematicae 41 (2023), pp. 20–54. DOI: 10.1016/j.exmath.2022.12.002. MR4557273. Preprint version: arXiv:2111.03230 [math.GT]. Corrected preprint: PDF.
Miscellaneous unpublished expository writing
  1. The lake where they had hidden the reflections: Quasi-Fuchsian groups and their embeddings [This was a talk in a reading group.]
  2. Knot knotes
  3. Apocrypha and ephemera on the boundaries of moduli space [More detailed notes on deformations of geometrically finite Kleinian groups are under preparation.]
  4. Uniformisation, equivariance, and vanishing: three kinds of functions hanging around your Riemann surface

Selected talks

Here is material (e.g. lecture notes, slides) from selected research talks I have given.
  1. (Upcoming) 11 December 2024: Deformations of 3-orbifold holonomy groups and applications, in the "Early Career Showcase in Low-Dimensional Topology" session at joint meeting of the NZMS, AustMS and AMS (Uni. of Auckland), slides will appear.
  2. (Upcoming) December 2024: Limit sets of cone manifolds (poster presentation), in the joint meeting of the NZMS, AustMS and AMS (Uni. of Auckland), PDF to appear.
  3. (Upcoming) November 2024: Combinatorial structures in trace polynomials of function groups, at the 8th Australian Algebra Conference in Canberra, slides.
  4. 12 November 2024: Two-bridge knots, genus two surfaces, and discrete groups with two generators, at the Hodgsonfest (Uni. Melbourne), slides.
  5. 24 July 2024: Is \( \mathrm{PSL}(2,\mathbb{Z}) \) discrete?, in the Topology Seminar (Monash University), slides.
  6. 10 May 2023: The dynamic in the static: Manifolds, braids, and classical number theory, in the RePS at Universität Leipzig, slides.
  7. 21 September 2022: What is a Kleinian group?, a talk aimed at undergraduates and beginning postgraduate students in the Australian Postgraduate Algebra Colloquium, slides, recording.
  8. 4 May 2022: Pictures of hyperbolic spaces, in the Discrete Mathematics and Geometry Seminar (TU Berlin), slides.
  9. 17 March 2022: Strange circles: The Riley slice of quasi-Fuchsian space, in Pedram Hekmati's seminar on moduli spaces (Uni. of Auckland), slides.
  10. 6 December 2021: The Farey polynomials, for the Groups and Geometry retreat on Waiheke Island, presentation slides.
  11. 2 December 2021: The Riley slice, contributed talk for the MATRIX workshop on groups and geometries, presentation slides, recording.
  12. 1 April 2021: Real varieties of spherical designs, in the Algebra and Combinatorics Seminar (Uni. of Auckland), presentation slides.
Click for a list of other talks, lecture notes, and other ephemera.
  1. November 2024: Variations on a theme of Wielenberg.
  2. July 2024: Reading group Hyperbolic Knot Theory, miscellaneous notes.
  3. 17, 18 July 2024: Guest lectures for MATHS 782 "Geometric Group Theory" (Uni. of Auckland), lecture notes for lecture 1b (surfaces), lecture notes for lecture 2a (conformal maps), miscellaneous problems, some books
  4. July-August 2023: Minicourse on knot theory and geometry (Uni. of Auckland), below.
  5. 16 March 2023: Connectedness of the Hilbert scheme in reading group of Javier and Angel, lecture notes.
  6. 17 to 20 January 2023: Apocrypha and ephemera on the boundaries of moduli space, a minicourse at the Uni. of Auckland (also 7 December 2022 at MPI), lecture notes.
  7. 10 October 2022: Uniformisation, equivariance, and vanishing: three kinds of functions hanging around your Riemann surface, at MPI, lecture notes.
  8. 3 August 2022: Reproducibility in Computer Algebra (MPI MIS), handouts for practical activity (event co-organised with Christiane Görgen and Lars Kastner).
  9. 15 July 2022: On the MathRepo page "Farey Polynomials", in the MathRepo: Data for and from your Research event (MPI MIS), slides.
  10. 24 May 2022: Projective varieties over \(\mathbb{C}\), in the Lorentzian polynomials day which I organised, slides.
  11. 27 April 2022: Strange circles: The Riley slice of quasi-Fuchsian space, in the Seminar on Nonlinear Algebra (MPI MIS), slides.
  12. 19 July 2021: The moulding of hyperbolic clay: Deformation spaces of Kleinian groups (Uni. of Auckland), presentation slides.
  13. 8 June 2021: Some properties of \(2 \times 2 \) matrices, in the UoA Dept. of Mathematics Student Research Conference, extended abstract, presentation slides.
    [This talk was one of the four winners of the Best Talk award, along with the talks of Isabelle Steinmann, Chris Pirie, and David Groothuizen Dijkema.]
  14. Very rough lecture notes for the graduate seminar I taught on Kleinian groups in Semester 1, 2021: PDF 1, PDF 2, further reading list, and post-mortem.
  15. 23 July 2020: Toric varieties (Uni. of Auckland), presentation slides.

Deformation spaces of rank two Kleinian groups

The Riley slice
A group on an elliptic pleating ray.
The dual complex to the Farey triangulation.

A rank two Kleinian group is a discrete subgroup of \( \mathrm{PSL}(2,\mathbb{C}) \) generated by two elements. If the group is non-elementary, then it is related in complicated and interesting ways to hyperbolic 3-orbifolds that have boundary at infinity consisting of a genus two Riemann surface.

A graph curve is an algebraic curve consisting of a number of thrice-marked spheres, each marked point corresponding to a node (a transverse intersection of two components). Each graph curve of genus two comes from a trivalent graph on two vertices and three edges. There are exactly two such graphs: the theta graph (each edge joins both nodes), and the handcuff graph (one edge joins the nodes, and the others begin and end on the same node). Both of these graphs are homotopy retracts of the genus two handlebody. Therefore there are only two graph curves of genus two.

On the edge of the deformation space of 3-manifolds with genus two surface at infinity there lie manifolds with the same configuration of spheres at infinity: pairs of thrice-punctured spheres with rank one cusps, with incidence graph a trivalent graph with two vertices and three edges (the incidence graph has vertices correspoding to topological components of the surface and edges corresponding to nodes).

By Thurston's ending lamination theorem (proved for this special case by Minsky and Miyachi), on the boundary of the 3-manifold space you get a different limit for each choice of embedding of the trivalent graph into the handlebody, and you can also take limits of such choices to get 'degenerate' orbifolds—the graphs might even be knotted! Conversely every boundary point arises in this way. So there is a very complicated map from the space of these boundary groups (which is basically a Teichmüller space, up to a small quotient) to the space of graph curves (which has two points). It turns out that this complicated map is basically reflecting the geometry of two-bridge links. Manifolds on the boundary that correspond to handcuff graphs arise from two-bridge links with two components, and manifolds corresponding to theta graphs arise from two-bridge knots. The knots do not live inside the deformation spaces, but they lie on tendrils of discrete groups that creep out beyond the moduli spaces.

The Riley slice is the space of Kleinian groups generated by two parabolic elements such that the quotient manifold is a Conway ball: a 3-ball with two arcs drilled out. Choosing a way of arranging these arcs into a rational tangle is equivalent to picking a simple closed curve on the boundary sphere; suppose that this curve is represented by a hyperbolic element \( W_{p/q} \) with trace \( \mathrm{tr}\, W_{p/q} < -2 \) in the holonomy group of the manifold (actually, you need to pick the correct component of the set of points where this word is hyperbolic, but this is immaterial for the time being). The boundary of the deformation space can be reached by smoothly deforming \( W_{p/q} \) until it is parabolic (trace equals \( -2 \)). Keep deforming \( W_{p/q} \) so that its trace decreases; the group is no longer discrete except sporadically, and these discrete groups correspond to replacing the parabolic arc with a cone arc. Eventually you reach \( \mathrm{tr}\, W_{p/q} = 2\), and in fact \( W_{p/q} = 1 \). You have now reached the fundamental group of the \( p/q \) 2-bridge link. The arc (which has now vanished to become a solid part of the knot complement) is an upper or lower unknotting tunnel for the knot; and the point on the boundary of the deformation space where this arc was parabolic corresponds to the manifold where both the knot and the unknotting tunnel have been drilled out as parabolic arcs from \( \mathbb{S}^3 \).

If this sounds interesting:

Cone manifolds and discrete geometry of indiscrete groups

An indiscrete deformation of a genus two function group with controlled cone angle. (Will appear in due course: poster presentation at the Joint Meeting in December.)

A zoo of Kleinian groups

The figure eight knot group. There are several useful 'zoos' of Kleinian groups with interesting properties; I collected several interesting groups and families of groups from a few sources, and you can find their limit sets on this page.

These images were produced using the Bella computational package for Kleinian groups. (Bella stood for Better Limit Set Drawer.)

A previous version of this package: Riley slice computational package (GitHub). With this earlier package I produced some animations, and some more limit sets. Some more visually impressive animations can be found on the website of Emily Dumas.

Minicourse on knot theory and geometry

A Seifert surface for the trefoil knot. In July 2023 I organised a minicourse on knot theory at the University of Auckland, focusing on the representation theory of holonomy groups. View the abstract or download the latest version of the notes.

Erratum: the Seifert surface of the figure eight knot drawn in the notes is not correct. A correct application of Seifert's algorithm and associated sketch may be found in L. Kauffman, On knots (Princeton), cited at that point in the notes.

There will be eight lectures over four weeks in 303.148 (for the first two weeks at least):

Wed, 2pmFri, 2pm
Classical knot theory5 Jul: Basics7 Jul: Fundamental group
Geometric knot theory12 Jul: Knot complements14 Jul: Hyperbolic invariants
Braids19 Jul: Two-bridge knots21 Jul: Braids and mapping class groups
Knot polynomials26 Jul: Classical28 Jul: Quantum

Josh Lehman gave the lecture on mapping class groups and Lavender Marshall gave the lecture on the Alexander polynomial.

Some useful links:

Lorentzian polynomials and algebraic geometry on matroids

If \( X \) is a sufficiently nice variety, the Chow group \( A^*(X) \) provides a homology theory on \( X \); in fact, it admits a ring structure coming from the intersection product. It turns out that such a theory can be made to work on more general spaces, for example one can define a Chow ring for matroids; then the various Hodge-type results (Poincaré duality, the hard Lefschetz theorem, and the Hodge-Riemann relations) carry over. Various nice polynomials can be defined with respect to this generalised Hodge theory and the associated cones of 'ample divisors' (which turn out to be submodular functions); these are the Lorentzian polynomials of Brändén and Huh.

A Day of Geometry and Lorentzian Polynomials

At the end of May 2022 there was a seminar at the Institut Mittag-Leffler on the work of Branden, Huh, Katz, and various other people on Lorentzian polynomials and the geometry of matroids; before this event on Tuesday 24 May, I organised a very informal Zoom workshop on some of the geometric background material.
Abstract. Even if you do not know what Lorentzian polynomials are, you may have heard of Minkowski volume polynomials, the polynomials of the form \( \mathrm{vol}(x_1 K_1 + \cdots + x_n K_n) \) where \( K_1,\ldots,K_n \) are convex bodies—and these are somehow the "canonical examples" of Lorentzian polynomials. The goal of the workshop is to give many different examples of Lorentzian polynomials arising in geometry. The talks will be very informal, non-technical, and have many pictures.

The final schedule was as follows (all times are CET). Many of the speakers have kindly allowed me to share their slides and/or lecture notes.

  1. 9.30am—Matroids and chromatic polynomials (Tobias Boege, MPI MiS): Slides
  2. 10:15am—Varieties over C and embeddings into projective space via elliptic curves (Lukas Zobernig, The University of Auckland): Slides
  3. 11:00am—Hyperbolic polynomials (Hisha Nguyen, V.N. Karazin Kharkiv National University)
  1. 1:30pm—Convex geometry & mixed volumes (Mara Belotti, TU Berlin): Slides
  2. 2:15pm—Projective varieties over \( \mathbb{C} \) (Alex Elzenaar, MPI MiS): Slides

Some background material

Spherical designs

A diagram of a spherical design.
A spherical \((3,3)\)-design in \( \mathbb{R}^3 \) of 16 vectors.
Spherical \((t,t)\)-designs are arrangements of points on the sphere (possibly with weights) which are spaced 'far apart from each other': they are finite sets in \( \mathbb{R}^d \) such that the integral over the sphere of each homogeneous polynomial of degree \(2t\) in \( d \) variables is equal to its average value on the set. There are generalisations of this definition to subsets of \( \mathbb{C}^d \) and \( \mathbb{H}^d \) (the \(d\)-fold product of the Hamiltonian quaternion algebra, not hyperbolic \(d\)-space!).

Optimal designs and near-designs

Shayne Waldron and I have a paper in preparation: Putatively optimal projective spherical designs with little apparent symmetry, computing various spherical designs in order to find those of minimal order; a large set of designs and near-designs are archived on on Zenodo at DOI: 10.5281/zenodo.6443356. You can look at the code used to generate these on GitHub; it uses the Manopt optimisation toolbox. This work was was funded in part by a University of Auckland Summer Research Scholarship (2019-20). You can view the final report for the scholarship.

Spherical designs and sums of squares

BSc(hons) dissertation and MSc thesis

Words about the Riley slice I completed my BSc(Hons) dissertation in 2020 under the supervision of Dr. Jeroen Schillewaert. My Master of Science thesis was completed in 2021-22 in the Department of Mathematics at the University of Auckland, under the supervision of Dist. Prof. Gaven Martin (NZ Institute of Advanced Study, Massey University) and Dr. Jeroen Schillewaert. For more information see the section on deformation spaces above.